1. Field of the Invention
The present invention relates to a magnetic recording medium in which information recording is performed by arrangement conditions of magnetization of a magnetic material, and in particular, to the magnetic recording medium capable of reproducing information with high recording density by generating magnetic wall displacement at the time of reproducing recorded information. Furthermore, the present invention relates to a method of producing such a magnetic recording medium.
2. Related Background Art
Various magnetic recording media are proposed as rewritable information recording media, and some of them are put to practical use. In particular, as a large-capacity recording medium capable of performing high-density recording, expectations have been placed in a magneto-optical medium for recording information by writing a magnetic domain on a magnetic thin film with thermal energy of a semiconductor laser and reading the information by using the magneto-optical effect.
Recently, as digitization of moving images has advanced, the magnetic recording media of a larger capacity are also in increasing demand. In order to make the density of the magnetic record media larger, an important point is how to reproduce high-density recorded information. In general, line recording density of an optical disk such as the magneto-optical medium significantly depends on a laser wavelength of a reproduction optical system and a numerical aperture of an objective lens. To be more specific, if a laser wavelength λ of a reproduction optical system and a numerical aperture NA of objective lens are decided, a diameter of a beam waist is decided so that 2 NA/λ or so becomes a detectable limit of a spatial frequency on reproducing a signal. Therefore, in order to realize a higher density of the optical disk, it is necessary to shorten the laser wavelength λ of the reproduction optical system and enlarge the numerical aperture NA of the objective lens.
However, there is a limit (limit of reproducing resolution) to the laser wavelength and the numerical aperture of the objective lens. Therefore, there are developed technologies for allowing recording density to exceed the limit of reproducing resolution by devising a structure and a reproducing method of the recording medium. As an example thereof, there are the magneto-optical medium and the reproducing method thereof capable of reproducing the information having recording density by exceeding the limit of the reproducing resolution of the reproduction optical system without reducing an amplitude of a reproduction signal, as described in Japanese Patent Application Laid-Open No. H6-290496.
This magneto-optical medium adopts a so-called domain wall displacement detection (DWDD) method and has a structure in which a domain wall displacement layer, a switching layer and a memory layer are sequentially stacked. The domain wall displacement layer has domain wall-resistant magnetism (which means a magnetic field capable of starting the displacement of the domain wall by gradual application of the magnetic field, which is different from generally called “coercive force”) relatively smaller than the memory layer, and the switching layer has a lower Curie temperature than the domain wall displacement layer and the memory layer.
Recording of the information is performed by forming the magnetic domains according to recorded data in the memory layer. The domain wall displacement layer has recording marks corresponding to the respective magnetic domains formed therein by switched connection with the memory layer. When reproducing the information, predetermined temperature distribution is formed on the medium with a heating beam, and it uses a domain wall displacement due to reduction in domain wall energy in conjunction with a temperature gradient thereof. To be more precise, the temperature gradient displaces the domain wall of the domain wall displacement layer existing in a boundary portion of the recording marks to a region where the switched connection with the memory layer is cut off, and detects a magnetic reversal in conjunction with this domain wall displacement as a change in a polarized state of reflected light. According to this reproducing method, a length of the recording mark in a track direction is extended in proportion to displacement of the domain wall at the time of reproduction. Therefore, it is possible to obtain a sufficient amplitude of the reproduction signal even if the density of the recording marks at the time of recording is enhanced.
As for the reproducing method accompanied by the domain wall displacement described above, it is desirable that the domain wall in a front boundary portion of the recording mark and the domain wall in a back boundary portion of the recording mark are separately and independently formed in substance in order to stabilize the domain wall displacement and improve reproducing characteristics. It is because, in a state of a recording mark surrounded by the domain walls existing on both sides of the recording mark and the front and back domain walls (here, the state of the recording mark surrounded by such closed domain walls is referred to as the state of the domain walls in the front and back of the recording mark being not separated), a new domain wall is generated on a side and domain wall energy increases when the front domain wall is displaced so that an energy gain due to the reduction in the domain wall energy in conjunction with a temperature gradient is canceled. However, as for an ordinary method of producing the medium by sequentially stacking the domain wall displacement layer, a switching layer and a memory layer on a substrate, a roughly uniform recording film is formed in a film plane direction. Therefore, it is difficult to form the recording mark with the front and back domain walls completely separated on such a recording medium.
Thus, as a technique capable of forming the recording mark with the front and back domain walls completely separated, there is a proposed method, for instance, of performing a process of transforming a magnetic film on track sides by annealing both sides of the track with a high-output laser after forming the recording film, and forming the recording mark astride that processed portion. In addition, there are the method of rendering the magnetic film on the track sides adequate by making use of a change in adhering conditions of the magnetic film on a groove side wall in conjunction with a surface form of the substrate such as a groove form, and the method of selectively removing the magnetic film on the groove side wall by etching.
In the case of the technique for causing some change to the magnetic film on the track sides as described above, a region where the change was caused can be used as a buffer region between the tracks. Therefore, the techniques are also effective in controlling cross light and crosstalk between the tracks. The domain wall displacement in the domain wall displacement layer is performed according to a temperature profile based on an isothermal line of domain wall displacement starting temperature of the temperature distribution formed on the medium at the time of reproduction. If performed by the above technique, there is also an effect that, by limiting an effective track width, the form of the domain walls in the front and back boundary portions of the recording mark become relatively linear so that its form consistency with the temperature profile is improved.
However, the above conventional technique is limited as to improvement of track density for the following reasons (1) to (3).
(1) The amplitude of the reproduction signal depends on the area of the recording mark occupying in a spot of a reproducing beam. Therefore, if the effective track width is rendered extremely smaller than a diameter of the spot of the reproducing beam, the area of the recording mark occupying in the spot becomes smaller and the amplitude of the reproduction signal is reduced. Thus, there is a limit to rendering the effective track width smaller.
(2) The track width has fluctuation of various frequency components in its length direction due to influence of processing accuracy and surface conditions. In particular, high-frequency fluctuation in a cycle equivalent to the thickness of the domain wall (for example, 50 nm) blocks the domain wall displacement.
To be more precise, if an energy change amount at the time of displacing the domain wall just by a minute distance dX is dE, a force acting on the domain wall is given by dE/dX. As the magnetic domain (corresponding to the recording mark) is recorded to the limit of the track width, a magnetic domain width changes if the track width changes. For this reason, the magnetic domain width becomes wide in a region where the track width is wide, so that a domain wall amount increases and the energy increases. Inversely, the magnetic domain width becomes narrow in a region where the track width is narrow, so that a domain wall amount decreases and the energy decreases. This energy change influences the domain wall displacement in the track direction. To be more precise, a force for blocking the domain wall displacement acts in a region where the track width becomes wide, and a force for promoting the domain wall displacement acts in a region where the track width becomes narrow. As the action of the force for blocking the domain wall displacement blocks stabilization of the domain wall displacement, the reproducing characteristics will deteriorate. The narrower the track width becomes, the greater the action of the force for blocking the domain wall displacement becomes.
(3) As a track pitch is an addition of the track width and the buffer region, it is thinkable to narrow the buffer region in order to improve the track density. However, the buffer region needs to secure a certain width to control the cross light and so on, and so there is a limit to narrowing the buffer region. In particular, in the case of the method of annealing it with the laser, it is difficult, because of a constraint of a laser spot size in the first place, to narrow the buffer region (anneal region) to that size or less.